2. Mangatawa catchment consents for earthworks, storm water ...

The Maranui catchment area in the model is 2.77 km 2 and includes the TEM catchment area and
excludes the Arataki residential area and a small portion of rural land north of Kairua Road. The
reason for including TEM catchment area was that the proposed mitigation measure will divert
water to the Maranui swale. The following is a summary of the catchment information:
Table 5 - Catchment peak flow rates (m 3 /s)
Catchment 10% AEP 2% AEP 1% AEP
P1 – central 28.67 54.74 66.50
P2 – east 14.92 25.62 31.64
P3 – west 6.79 10.76 15.39
Maranui 11.50 22.66 27.69
TEM
- A
A – TEM has an extended catchment that essentially contributes to two catchments and therefore does not have a distinct primary flow hydrograph.
See Appendix C for the runoff hydrographs for the following catchments P1, P2 and P3.
2.4 Hydraulic Model
2.4.1 Sources of Information
The following information was sourced to assist in producing a hydraulic model that is
representative of the catchments and meets the mitigation objectives of the project:
• MIKE11 hydraulic model, Opus International Ltd
• Additional cross-section information of Mangatawa drain, Opus International Ltd
• LiDAR data of the areas surrounding the drains, TCC
• Papamoa East as-built information of roads and stormwater data, TCC
• Maranui swale, culvert and treatment pond design information, Shrimpton and Lipinski Ltd
• Maranui swale, as-built information, TCC
• MPBI conceptual development plans, Harrison Grierson Consultants Ltd
• Isthmus proposed Maranui treatment pond and Mangatawa Drain
2.4.2 Mitigation Objectives
The primary aim of the parties was to reduce the number of highway closures and the volume of
water spilling into the Maranui and Wairakei catchments without negatively affecting the
surrounding environment. The parties aim to achieve this through the provision of engineering
solutions, e.g. additional storage and conveyance in the Mangatawa Drain, Maranui swale system.
A comprehensive list of work is provided in Section 2.1.
2.4.3 Parameters Assessed
The model was used to assess the following parameters:
• Flooding across the highway
• Maximum flood water levels in the catchments for the 1% AEP event
• Residence time of water in each catchment for the 1% AEP event
• Water levels, peak flow rates and velocities along the following two drainage paths:
Upper and lower Mangatawa Drain defined by the rail bridge
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Maranui pond floodway corridor
Beca // 17 March 2009 // Page 10
3932036 // NZ1-1604459-15 0.15 Rev A